Translucent screen



Patented Jan. 30, 1945 TRANSLUCENT SCREEN Bernard M. Bodde, Los Angeles,Calif.

Original application November 2, 1942. Serial No. 464,263. Divided andhis application August 2, 1943, Serial No. 49 ,033

1 Claim.

The invention relates to the translucent type of screen suitable for theprojection of still or motion pictures on one side thereof, for viewingor photographing on the opposite side.

A common diiiiculty encountered in the projection of pictures upon atranslucent screen is,-

unless special precautions are taken, the picture has a "hot spot due toa higher value of light intensity at the center of the screen than atthe edges.

Heretofore, this hot spot problem has been approached from thestandpoint of bending the unwanted amount of light rays away from thescreen abaxially of the projection axis. According to the presentinvention, I approach this problem from an entirely different standpointand stop the unwanted rays at the screen, while nevertheless usingtransparent material therefor. Preferably this stopping of the unwantedlight trays at the screen is graduated along the screen in a manner toovercome the hot spot. By stopping the desired amount of the light raysat thescreen, I obtain a sharp picture on the projection side of thescreen, and a picture which has the appearance of substantial depth.

For further details of the invention reference may be made to thedrawing wherein Fig. 1 illustrates diagrammatically a matrixcrystallinematerial, the density thereof increasing towards the center of thescreen.

Fig. 3 is an enlarged plan view with parts bro:

ken away of a corner of the screen of Fig. 2.

Fig. 4 is a sectional view on line 4-4 of Fig. 3. As explained in myformer patents, I prepare a translucent picture projection screen lhaving a flexible transparent base I of plastic material such ascellulose acetate, ethyl ce1lulose,.

or other plastic. The transparent sheet I may be formed by spraying asolution thereof in a suitable solvent, by means-of the spray 2 on theun-' derside of a horizontal matrix 3. Multiple coats are applied, untilthe base I is, for example, .015 inch thick,or at least thick enough towithstand stretching flat, without thereby stretching the coating 6 onthis base, for reasons which will be explained. For small screens, thebase I may be prepared by shaving a block of the plastic material. WhileI prefer to make the base I by spraying on the underside of the matrix3, this matrix may be arranged in any other suitable position.if'desired. Regardless of the manner in which the base I is obtained, Iarrange it to receive the coating of this invention, and for thispurpose if the base I is made by spraying on the matrix 3, it may beleft on this matrix and the coating sprayed thereon, or it may bestripped from matrix 3, or a shaved sheet maybe taken and arranged inany suitable position to receive the coating.

To prepare the coating of this invention, preferably I take a solutionof plasticinaterial, viz. the same material used to make the base i andmix therewith a solution of material adapted to crystallize to formmicroscopic crystals adapted to polarize light, and apply the resultingcoating to the base I without stretching the base R either while thecoating is drying, or thereafter, whereby a random orientation of themicroscopic crystals is obtained when the crystals form in situ on thebase I. tion of light passing through the screen. Preferably I preparethis polarizing solution in ap= proximately the following manner andproportions. To six grams of quinine bisulphate, or other members of thequinine family, add 200 cc. of butyl alcohol, heat and stir to dissolvethe quinine until it becomes viscous or thick like molasses. While it ishot, add thereto about an equal quantity of approximately two grams ofiodine sulphate dissolved in 40 cc. of ethyl alcohol, mix and allow tocool and the mass will become of the consistency of gelatine. The iodinedelays the crystallization of the quinine. Then take 1% or 2% of thegelatine-like mix and add to 99% or 98% of the screen material such asethyl cellulose, or cellulose acetate dissolved in a solvent, mixthoroughly and spray this coating material with a spray such as .1, ontothe base I. Preferably the coating material which is described is firstuniformly sprayed on the base I, very thin, for example to a thicknessof .0001 inch. Then beginning at a place inside the margin of the screenas indicated at the rectangle spray another thin coating and thenbeginning at a greater distance from. the margin as indicated by therectangle 5 spray another coating, etc. to gradually increase thethickness of the coating 6 as the center of the screen is approached.The thickness of the coating 6 at the center of the screen may be of theorder of live times as thick as it is at the edges, namely .0005 inch.

As the polarized coating material is largely composed of the samematerial as the base I,

This results in random polarizathis coating 6 integrally unites with thescreen to form a coating which is flexible like the base I, and thiscoating is transparent.

On examining the coating thus produced, under a microscope withmagnification about 700, the quinine crystals look like telephone poles,and their arrangement is entirely random, that is, like a jumbled massof telephone poles indicated at I! in Fig. 3, with the thickness of thepile oi telephone poles or microscopic crystals, increasing towards thecenter of the screen as shown. However, the proportions above indicatedare such that the crystals are separated so that light can pass betweenthem without being polarized, such interstices decreasing in size as thethickness of the crystal pile increases towards the center of thescreen. However, the light that strikes a crystal at the bottom of thepile is polarized in one direction and such light, on striking a crystalabove it which lies at an angle thereto, can pass through the uppercrystal in an amount depending upon the angle between the two crystals.If they are at right angles to each other, no light will pass where theupper crystal intersects the light from the lower crystal. The areas inwhich the light is transmitted through the coating are very small incomparison to the size of the magnified silver grain of the picture asit appears on the screen and hence whatever appearance of grain may bein the projected picture is not increased by reason of the use of thiscoating.

If the base I were not employed and if the coating 6 were stretched inone direction either while it is drying, or after it has dried, thiswould have a tendency to orient all of the crystals in one directionwhereby the coating as a whole would polarize all of the light reachingit, in one direction. While this feature may be useful in othersituations, I take pains to avoid it, and

hence allow the coating 6 to dry and crystallize on the base I withoutstretching it and also employ a base i which is many times thicker thanthe coating and thick enough to permit the screen to be stretched fiatwithout stretching the coating.

A screen made according to the present invention has been constructedand tested and it has been found that it stops the unwanted quantity oflight at the screen and the picture projected on the screen has theappearance of substantial depth. Also the image of the projected pictureis formed on the screen In even though its base i and coating 5 are bothtransparent.

If desired, a small quantity of suitable thinner or solvent may beemployed to cut the gloss and resulting glare on the backside I of thescreen.

The screen 10 as disclosed in my above patents. may be mounted by meansof a flexible cord II in a suitable framework l2, Or any other suitablemounting may be employed.

This application is a division of S. N. 464,263 filed November 2, 1942,for Translucent screen manufacture.

Various other modifications may be made in the invention withoutdeparting from the spirit of the following claim.

I claim:

A translucent picture projection screen comprising a sheet ofsubstantially transparent material, and a coating thereon of plasticmaterial having embedded therein a random array of stacks of microscopiccrystals adapted to polarize light, said crystals at the edge of saidsheet being spaced far enough apart in a direction across said sheet toadmit light through said sheet between said crystals, the intersticesbetween said crystal stacks becoming smaller in size toward the centerof the screen.

BERNARD M. BODDE.

